Monday, June 4, 2012

Thigmonasty Isn't Nasty

Sensitive Briar- Click to enlarge
As a kid running around a dry rocky field in Kansas, I was delighted in finding sensitive briar.  The idea that a plant could respond quickly to touch was exciting - and still is.  I was busy watching the leaves and didn't notice the tiny thorns or beautiful flowers at that time.

Sensitive Briar, Mimosa nuttallii (a.k.a. Cat's Claw, Nuttall's Sensitive Briar, Bashful Briar, Shame-boy, Devil's Shoestrings) is a common plant on some prairies and glades and can be found on disturbed soils.  While some gardeners are ambivalent about its spreading nature, cattle love it and its seeds are high in protein.

Sensitive briar was named the 2010 Wildflower of the Year in Kansas.  A Missouri University cynic might snidely say that Kansas didn't have much to choose from, but for my money it was really cool.  The part that excited me was the closing of the leaves to touch.  Well OK, there weren't a lot of exciting things to do in Kansas back then and I wasn't the swiftest kid on the block, but it could keep me busy for a while.  By the time I was done, there wouldn't be an open leaf in 50 feet.

Before touch
After touch
 As a kid I was excited by its thigmonasty, even though I had never heard of it.  Thigmonasty is the mechanism which closes the briar's leaves as though they had little muscle cells.  The "nasty" part refers to nastic movements of plants which are non-directional responses to stimuli (e.g. temperature, humidity, light, and touch).  Another good example is the closing of Venus Fly Trap.

Pulvinus shrink at bottom
So how do the plant leaves manage to flex shut so quickly?  Thigmonasty in Wikipedia provides the details but a simplified answer is that touch (finger, rain, etc) stimulates some of the pulvinus cells at the base of the leaflet to extrude water from the long cells on one side, making them shrink.  The cells on the other side remain full,  creating more pressure on the outside of the curve, causing the leaflet vase to flex.

Redvine- Wikimedia
Thigmotropism is another type of plant response to pressure.  In this case it is directional rather than non-directional.  "Positive thigmotropism" refers to the plant moving in the direction of the pressure and is responsible for the wrapping of tendrils of vines around a stem or stalk.

There is also "negative thigmotropism" causing roots pressing against one of our Ozark rocks to turn away in another direction.  Now if we could incorporate that into an Ozark shovel, botanical science could be a money maker.

More on plant responses from Natural History magazine is at Do Plants Have Brains

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